[CH2:1]([O:3][C:4]1[CH:5]=[C:6]2[C:11](=[N:12][C:13]=1[CH3:14])[N:10]=[CH:9][C:8]([C:15]([O:17]CC)=[O:16])=[C:7]2[OH:20])[CH3:2].[OH-].[Na+]>O>[CH2:1]([O:3][C:4]1[CH:5]=[C:6]2[C:11](=[N:12][C:13]=1[CH3:14])[N:10]=[CH:9][C:8]([C:15]([OH:17])=[O:16])=[C:7]2[OH:20])[CH3:2]A mixture of ethyl 6-ethoxy-4-hydroxy-7-methyl-1,8-naphthyridine-3-carboxylate (10.0 g), 5M sodium hydroxide solution (200 ml), IMS (64 ml) and water (64 ml) was boiled under reflux for 2 hours. The mixture was cooled to ambient temperature and then filtered. The residue was dissolved in water, acidified to pH 5 with glacial acetic acid and filtered. The residue was triturated with hot IMS, cooled and filtered to give 6-ethoxy-4-hydroxy-7-methyl-1,8-naphthyridine-3-carboxylic acid, m.p. 259°-261° C.
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